Field of the Invention
The present invention relates to methods for generating combustion in furnaces and burner assemblies therefore which include a refractory block, a fuel supply system and an oxidant supply system, the assemblies being configured to generate a flame downstream of the refractory block.
The present invention is particular suited for use in melting processes. It is notably, but not exclusively, suited for use in secondary metal, melting, in particular secondary aluminium melting, and ladle preheating.
Related Art
Melting processes generally comprise several phases or stages:                a loading or charging phase in which the solid raw material is fed to the furnace,        a melting phase in which the solid raw material is melted to form molten material,        a maintenance, fining or refining phase in which the molten material is maintained in the molten state until it reaches a required level of homogeneity,        a tapping or discharge phase, in which the refined molten material is removed from the furnace for further processing.        
Different requirements of temperature, energy, etc. apply to the melting and fining phases. The most power or energy (per weight of material) is required during the melting phase, whereas less power or energy (per weight of material) is required during the fining phase.
Ladles can be used to carry molten material, in particular molten metal, from the melting furnace to a downstream installation, such as a ladle refining station or a casting station. These ladles are usually preheated to minimize thermal shock and damage to the refractory lining and to reduce temperature drop in the ladle.
Ladle preheating processes likewise generally comprise several phases or stages:                An initial or primary phase of heating up the ladle vessel to an elevated temperature,        A holding or temperature equilibrating phase when the ladle vessel is maintained at an elevated temperature, allowing a uniform temperature distribution throughout the refractory material        
The driving forces for cost reductions in melting industries, such as secondary melting industries, are mainly focused along two axes: the reduction of operation costs and the improvement of the process control. Important parameters are:                reduction of energy costs,        increase of productivity;        improvement of the process control, which includes:                    better stability of the atmosphere in furnaces;            larger abatement of pollution, such as NOx and black fumes containing impurities like dusts.                        
A specific parameter for secondary aluminium smelters is the reduction in the formation of dross (the mixture of salt, dirt, aluminium oxides and entrapped metallic aluminium that forms at the surface of the molten aluminium).
During the melting phase, which is the most energy consuming, it would be beneficial to use an oxidant with high oxygen content, so as to achieve a higher heat transfer to the raw material by radiation, thus accelerating the melting process, increasing energy efficiency and reducing energy consumption.
During the fining phase, in which inter alia temperature homogenization of the molten material takes place, less energy is required and fuel consumption is drastically lower. During this phase, lower oxygen participation (i.e. a lower oxygen concentration in the oxidant) could be used to minimize the operation costs, depending on the respective prices of fuel and oxygen.
An aluminium smelting process in which oxycombustion is used during the melting phase and in which air combustion is used during the holding phase is described in DE-A-10046569.
Furthermore, as will be discussed below, other benefits may be achieved in certain melting processes, such as secondary aluminium smelting, by using, during the fining phase, an oxidant, such as air, with a lower oxygen concentration.
In the case of ladle preheating, it is beneficial in the primary phase to use an oxidiser with high oxygen content, thus making it possible to reach the desired temperature as fast as possible and consequently to reduce the overall energy consumption. During the second temperature equilibrating phase, it can be beneficial to use a cheaper oxidiser with low oxygen content such as air since the energy requirements for this part of the process are lower. The operation costs can be minimized, depending on the respective prices of fuel and high oxygen oxidizer.
One family of prior art burner apparatus is disclosed in EP-A2-0754912, to which the reader is referred for further background information. In this state-of-the-art system, fuel and oxidant are introduced into the furnace through separate cavities in the burner assembly so that the fuel burns with the oxidant in a wide luminous flame, and whereby the combustion of the fuel with the oxidant generates reduced quantities of nitrogen oxides (NOx). Such a prior art burner apparatus provides both good energy efficiency and reduced production of pollutants (NOx). One problem with the apparatus described in EP-A2-0754912 is that it is limited to operation with an oxidant in the form of a gas having an oxygen molar concentration of at least 50%. This minimum oxygen requirement limits the flexibility of the apparatus.
US-A-2001/023053 discloses a burner block assembly which permits oxy-fuel, air-fuel, or an oxygen enriched air-fuel operation without replacing the burner block. However, combustion must be interrupted and the burner inlet arrangement must be modified when switching from oxy-fuel operation to air-fuel operation or to oxygen enriched air-fuel operation. US-A-2003/0157450 discloses a specific embodiment of this type of burner block assembly for the combustion of preheated fuel with preheated oxidant. According to one aspect of said embodiment, the burner block assembly comprises a conduit adapted to convey preheated oxidant and which extends through a plenum adapted to pass ambient temperature fluid into the annular region of the plenum surrounding the preheated oxidant conduit, thereby minimizing thermal stresses on burner parts and net heat loss. The ambient temperature fluid passing into the annular region surrounding the preheated oxidant conduit may itself be an oxidant and, in particular, an oxidant of different composition than the preheated oxidant.
U.S. Pat. No. 4,547,150 discloses a burner assembly with a central fuel injector and a co-axially surrounding oxidant injector, whereby the oxygen content of the oxidant can be varied from no oxygen enrichment (air-fuel combustion) to different levels of oxygen enrichment.
DE-A-10046569 and US-A-US2002192613 disclose pipe-in-pipe burners for use with two different oxidants with concentric fuel and oxidant injectors and a fuel-oxidant premixing chamber downstream of the fuel injector.
JP-A-2000146129 discloses a variable rate oxygen enrichment burner with a central fuel gas path and a coaxially surrounding air supply path, and a plurality of tube bodies surrounding the fuel gas path and positioned within the coaxial air supply path.